Advances in microfluidic research has enabled lab-on-a-chip (LoC) technology to achieve miniaturization and integration of biological and chemical analyses to a single chip comprising channels, valves, mixers, heaters, separators, and sensors. These miniature instruments offer the rare combination of faster, cheaper, and higher-precision analyses in comparison to conventional bench-scale methods. LoCs have been applied to diverse domains such as proteomics, genomics, biochemistry, virology, cell biology, and chemical synthesis. However, to date LoCs have been designed as application-specific chips, which incurs significant design effort, turn-around time, and cost, and degrades designer and user productivity.
A significant operation in almost all assays performed on LoCs is the mixing of one or more fluids. Typically, assay protocols dictate that fluids are mixed in certain ratios, and, in some cases, in certain absolute volumes. To achieve different mixing ratios, current LoCs typically employ input channels of different dimensions, where the ratio of the dimensions is equal to the desired mixing ratio outcome. These channels then feed into a common, larger channel where mixing by diffusion takes place. To achieve variable volume mixing, current LoCs use external metering techniques to carefully measure the required fluid volumes prior to their mixing. In either case, the volume of each fluid used in the mixing process is variable.
Programmable LoCs (PLoCs) have been developed which are flexible and not limited to specific applications. These PLoCs are also capable of automatically conducting assays. In a PLoC, the assays are programmed in a high-level language and are compiled to automatically run on a general or multi-purpose microfluidic chip.
PLoCs have a general layout with a general set of microfluidic components. All channels have equal, fixed cross-sectional dimensions. Fluid flows in discrete volumes rather than in a continuous-flow approach. Therefore, the existing techniques to achieve variable ratio mixing cannot be applied to conventional PLoCs. Instead, current PLoC mixers must use a fixed volume approach to mixing where a mixer is completely filled with a defined total volume of two liquids before mixing can take place. Furthermore, because the fluids to be mixed in absolute volumes may be generated in intermediate assay steps, external fluid metering to achieve variable volume mixing is not possible. Accordingly, it would be desirable to provide a PLoC that is capable of variable volume mixing.
One issue with providing a variable volume mixing system in a PLoC mixing systems is the introduction of air bubbles into the mixer. Air bubbles can impede mixing efficiency by keeping two fluids separated from one another, thus inhibiting the mixing process. Accordingly, it would also be advantageous if such a variable volume PLoC mixing system were capable of effectively handling air bubbles in the mixer.
Yet another issue with mixing systems in PLoC arrangements is fluid volume management. The issue of fluid volume management arises because fluids have a fixed total volume, and the use of a fluid in one instance depletes the total volume, leaving less fluid for later uses of the fluid. If there are many uses of a fluid, the given volume of the fluid must be distributed carefully among the uses to prevent execution from running out of the fluid before all of the uses occur. This distribution poses a challenge when the uses require different proportions of volumes as is the case when a fluid is mixed with different other fluids in different ratios (e.g., one use for a fluid is in a mix ratio of 1:2 while another use for the same fluid is in a mix ratio of 1:10). Dealing with such distributions is further complicated by low-level, implementation-dependent details of the fluidic hardware, such as maximum capacity (of reservoirs and functional units) and minimum fluid transport resolution (imposed by the fluid transport/handling hardware). Forcing the programmer to handle these constraints would diminish the practicality of PLoCs. Consequently, it would be advantageous to handle this issue automatically using a combination of the compiler and run-time system.
One proposed method for dealing with fluid volume management issues is a reactive approach for volume management called regeneration. Regeneration allows the fluid to run out and re-generates the fluid just before the next use by re-executing the code fragments that produce the fluid (i.e., the backward slice). While elegant in theory, regeneration may place a high or unbounded demand on LoC resources. Repeating unbounded resources through virtualization is feasible in conventional computers, but microfluidic technology is not yet at that level of maturity. Even when regeneration is feasible, regeneration re-executes fluidic instructions (in the fluidic datapath) which are slow and are likely to incur overhead (PLoCs use a heterogeneous organization where the datapath is fluidic and control is electronic and orders-of-magnitude faster). In view of the foregoing, it would be desirable to provide a more pro-active approach to fluid volume management in order to reduce the chances of running out of a fluid. It would be advantageous if the improved method of fluid volume management could largely avoid regeneration's overhead but maintain some of the other advantages of regeneration.
Therefore, because of all the above stated shortcomings, it would be desirable to provide an improved apparatus and method for the transport and metering of variable volumes of fluids in a LoC arrangement.